Precious metal three-way catalysts are generally used as a means for removing pollutants from the exhaust gas of an internal combustion engine. These three-way catalysts remove CO, HC, and NO.sub.x simultaneously from engine exhaust gases under stoichiometric conditions. However, under lean fuel conditions, which are desired for optimal fuel efficiency, the three-way catalyst is ineffective for the removal of NO.sub.x. Accordingly, to achieve NO.sub.x control fuel lean conditions, exhaust after-treatment systems have included a lean NO.sub.x trap (LNT). Presently, however, the performance of NO.sub.x trap technology is limited in several respects. NO.sub.x trap performance is affected by the operating temperature and requires a relatively narrow temperatureoperating window. At temperatures outside this window, the device may not operate efficiently and NO.sub.x emissions can increase.
Both three-way catalysts and lean NO.sub.x traps (LNT) are generally inefficient at ambient temperatures and must reach high temperatures before they are activated. Typically, contact with high-temperature exhaust gases from the engine elevates the temperature of the catalyst or LNT. The temperature at which a catalytic converter can convert 50% of CO, HC, or NO.sub.x is referred to as the "light-off" temperature of the converter.
During start up of the engine, the amount of CO and HC in the exhaust gas is higher than during normal engine operation. While a large portion of the total emissions generated by the engine are generated within the first few minutes after start up, the catalysts are relatively ineffective because they will not have reached the "light-off" temperature. In other words, the catalysts are the least effective during the time they are needed the most.
A reduction in harmful emissions is a constant concern for the environment. Another concern is increased fuel efficiency. It is possible to save fuel by operating in a fuel lean A/F mode and/or on less than all cylinders during certain operating conditions. The variable displacement engine has a controller that disables selected cylinders in the engine, causing the engine to have a decreased effective displacement, through control of a plurality of engine cylinder valve deactivators.